The invention pertains to the field of controlled-rubber vulcanization art. The applicable U.S. patent classification defines the invention as "retarders. "
In the manufacture of vulcanized rubber products, crude rubber is combined with various other ingredients such as fillers, accelerators, and antidegradants to alter and improve processing of the rubber and to improve the properties of the final product. The crude rubber is put through several steps in the plant before it is ready for the final step of vulcanization. Generally, the rubber is mixed with carbon black and other ingredients except the vulcanizing agent and accelerator. Then the vulcanizing and accelerating agents are added to this masterbatch in a Banbury mixer or a mill. Scorching, viz., premature vulcanization, can occur at this stage of the processing, during the storage period before vulcanizing, and during the actual vulcanization. After the vulcanizing and accelerating agents are added, the mixture of crude rubber is ready for calendering or extruding and vulcanization. If premature vulcanization occurs during the storage of the crude mixture or during processing prior to vulcanization, the processing operations cannot be carried out because the scorched rubber is rough and lumpy, consequently, useless. Premature vulcanization is a major problem in the rubber industry and must be prevented in order to allow the rubber mix to be preformed and shaped before it is cured or vulcanized.
There are several reasons offered for premature vulcanization. The discovery of the thiazolesulfenamide accelerators constituted a major breakthrough in the vulcanization art, because thiazolesulfenamides delayed onset of the vulcanizing process; but, once it started, the built-in amine activation of the thiazole resulted in strong, rapid curing. Mercaptobenzothiazole is a valuable organic vulcanization accelerator but by present standards would be considered scorchy. It has been largely replaced by the delayed-action accelerators. The development of high pH furnace blacks which lack the inherent inhibiting effect of the acidic channel blacks and popularity of certain phenylenediamine antidegradants which promote scorching have placed increasingly stringent demands on the accelerator system.
Retarders have long been available to rubber compounders. These include N-nitrosodiphenylamine, salicylic acid, and a terpene-resin acid blend. See Editors of Rubber World, "Compounding Ingredients for Rubber," 125-128 (3rd. Ed., 1965). Acids as retarders are generally ineffective with thiazolesulfenamide accelerators or adversely affect this vulcanizing process. Nitrosoamines as retarders are only of limited effectiveness with thiazolesulfenamides derived from primary amines.